Water Flashcards
Name two chemical tests for the presence of water and their colour changes
Anydrous copper sulfate white –> blue Anydrous cobalt chloride blue –> pink
What is hard water?
Water that does not lather easily with soap.
Which ions are dissolved in hard water?
Calcium or Magnesium
What forms when hard water reacts with soap?
An insoluble scum called calcium stearate Sodium stearate + calcium ions —> calcium stearate + sodium ions
What is temporary hard water?
Hard water which can be softened by boiling.
Give four physical properties of water?
Liquid at room temp and pressure Colourless Melting point 0°C Boiling point 100°C
How does temporary hard water form?
Carbon dioxide dissolves in rain water forming a weak carbonic acid. CO2 + H2O –> H2CO3 Carbonic acid falls on limestone rock and forms soluble calcium hydrogen carbonate. H2CO3 + CaCO3 –> Ca(HCO3)2 Calcium ions are in the water
What happens to temporary hard water when it is heated up?
It thermally decomposes forming insoluble calcium carbonate Ca(HCO3)2 –> CaCO3 + H2O + CO2
What is permanent hard water?
Hard water which can not be softened by boiling.
Which calcium or magnesium compounds form permanent hard water?
Calcium chloride or magnesium sulfate
Give four advantages of hard water
Tastes good Contains calcium ions needed for strong bones and teeth Better for tanning leather Better for brewing beer
Give four disadvantages of hard water
Wastes soap Formation of scale in pipes and kettles Expensive - dishwasher salt required to prevent lime scale in dishwashers
Give three methods of softening hard water
Boiling (temporary hard water only) Washing soda Ion exchange column
Explain how washing soda removes hardness in water.
The calcium ions from the hard water combine with carbonate ions from the washing soda (sodium carbonate) to form insoluble calcium carbonate. Ca2+ + CO32- –> CaCO3(s)
Explain how an ion exchange column removes hardness in water.
The ion exchange column contains a resin with sodium ions attached. When the hard water is poured over the calcium ions are replaced by sodium ions.
What effect does hard water have on detergents? e.g. Cillit Bang
No effect
What is a precipitation reaction?
A reaction where ions from two solutions react together to make an insoluble substance.
What does a deliquescent substance do?
Absorbs moisture and dissolves in the moisture making a solution.
What is a dessicant?
A substance which absorbs moisture from the air.
Give an example of a dessicant substance and where it is used.
Silica gel used in packaging of electrical equipment e.g. Camera
Give an example of a deliquescent substance
Calcium chloride used in the rusting experiment to remove moisture.
Why is fluoride added to water supplies.
Increases the resistance of teeth to attack from acid.
Give some advantages of adding fluoride.
Reverses the effects of loss tooth enamel Increases resistance of teeth to acid decay Ensures all the population has access to the benefits of fluoride
Give some disadvantages of adding fluoride to water
High concentration leads to dental fluorosis Difficult to monitor dosage as people drink differing amounts of water or eat foods high in fluoride Costly
